“I used to believe absolutely everything that Bill just presented,” the scholar Bart D. Ehrman once said during a 2006 debate with the conservative theologian William Lane Craig. “He and I went to the same evangelical Christian college, Wheaton, where these things are taught. … I used to believe them with my whole heart and soul. I used to preach them and try to convince others that they were true. But then I began … looking at them deeply myself.”

Ehrman, in other words, is no longer an evangelical, or even a Christian. Although he’s written a number of valuable books on the shortcomings of fundamentalist readings of Scripture, not every enemy of fundamentalism has approved. On his popularblog, Ehrman has occasionally responded to personal attacks by the atheist crowd, who do not share his considered equanimity. In 32 years, he’s managed to write or edit more than 30 books, while also pausing to debate Christology with Stephen Colbert.

The field of New Testament studies has never been a reliable starting point for scholars seeking publishing superstardom. One explanation for this is the subject matter itself. A true understanding of the forces that shaped Christianity — seemingly familiar but in fact highly arcane — requires the ability to synthesize and express deep learning in a dozen interlocking subjects. Ehrman, who considers himself a historian but has done extensive work in textual criticism, has managed to achieve his remarkable renown by writing a string of best sellers that skillfully mine and simplify his more scholarly work.

That may sound pejorative, but it’s not. Ehrman’s outreach to a popular audience — among whom I happily include myself — is wholly to the good, if only because throughout history average Christians have proved oddly unwilling to dig into the particularities of their faith, beyond familiarizing themselves with a few tentpole doctrines. They share this reluctance with one of Christianity’s most spectacular converts, the Roman emperor Constantine, who credited his victory at the Milvian Bridge in A.D. 312 to the auspices of the Christian deity, despite not knowing much about Christianity, including the degree to which it was riven by sectarian disagreement. The following year, Constantine co-issued the Edict of Milan, granting Christians the right to practice their faith unmolested.

In “The Triumph of Christianity,” Ehrman describes the Edict of Milan (which was neither an edict nor written in Milan) as the Western world’s first known government document to proclaim the freedom of belief. At the time, Ehrman notes, “Christianity probably made up 7 to 10 percent of the population of the Roman Empire.” A mere hundred years later, half the empire’s “60 million inhabitants claimed allegiance to the Christian tradition.” Ehrman declares, without hyperbole, “That is absolutely extraordinary.”

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Over the centuries, countless books have been written to explain this, a great many of them by Christian writers and scholars who take the Constantinian view: Their faith’s unlikely triumph was (and is) proof of divine favor. Interestingly, pagan advisers argued in vain to the first Christian Roman emperors that pagan beliefs had been what won the empire favor in the first place. When the emperor Valentinian II removed the altar of the goddess Victory from the Roman Senate house in A.D. 382, for instance, a pagan statesman named Symmachus reminded him, “This worship subdued the world.”

Very little about the historical triumph of Christianity makes sense. When Constantine converted, the New Testament didn’t formally exist and Christians disagreed on basic theological concepts, among them how Jesus and God were related. For those living at the time, Ehrman writes, “it would have been virtually impossible to imagine that these Christians would eventually destroy the other religions of Rome.” Some saw glimmers of danger, however. An otherwise unknown pagan philosopher named Celsus wrote a tract called “On the True Doctrine” that attacked Christians’ penchant for secrecy, refusal to partake in public worship and naked appeals to “slaves, women and little children.”

The great appeal of Ehrman’s approach to Christian history has always been his steadfast humanizing impulse. In his superb book “The Orthodox Corruption of Scripture,” which concerns textual variants in early Christian texts that were driven by theological agendas, Ehrman argues that these corruptions weren’t typically the product of willful obfuscation but rather the work of careful scribes trying to make sense of often perplexing language, imagery and traditions. Ehrman always thinks hard about history’s winners and losers without valorizing the losers or demonizing the winners. The losers here, of course, were pagan people.

Ehrman rejects the idea that Constantine’s conversion made much difference; the empire, he writes, would most likely have turned Christian in time without him. So how did Christianity triumph? To put it plainly, Christianity was something new on this earth. It wasn’t closed to women. It was so concerned with questions of social welfare (healing the sick, caring for the poor) that it embedded them into its doctrines. And while there were plenty of henotheist pagans (that is, people who worshiped one god while not denying the validity of others), Christianity went far beyond henotheism’s hesitant claim upon ultimate truth. It was an exclusivist faith that foreclosed — was designed to foreclose — devotion to all other deities. Yet it was different from Judaism, which was just as exclusivist but crucially lacked a missionary impulse.

Ehrman, summarizing the argument of the social historian Ramsay MacMullen, imagines a crowd of 100 pagans watching a persuasive Christian debate an equally persuasive adherent of the healing god Asclepius: “What happens to the overall relationship of (inclusive) paganism and (exclusive) Christianity? … Paganism has lost 50 worshipers and gained no one, whereas Christianity has gained 50 worshipers and lost no one.” Thus, Christian believers go from roughly 1,000 in A.D. 60, to 40,000 in A.D. 150, to 2.5 million in A.D. 300. Ehrman allows that these raw numbers may look “incredible. But in fact they are simply the result of an exponential curve.” At a certain point, math took over. (Mormonism, which has been around less than 200 years, has seen comparable rates of growth.)

Ehrman quotes a valuable and moving letter from a devout pagan named Maximus, which was written to Augustine near the end of the fourth century: “God is the name common to all religions. … While we honor his parts (so to speak) separately … we are clearly worshiping him in his entirety.” But when pagan intellectuals decided to confront Christianity on its exclusivist terms — “We believe in one God as well!” — they effectively stranded themselves on their own 20-yard line. The heart-rending pagan inability to anticipate the complete erasure of their beliefs gave Christianity one clear path to victory.

And yet, when the caliga was on the other foot, Christians had different opinions about religious oppression and compulsion. Many of Christianity’s earliest apologists wrote of their longing to be left alone by the Roman state. Here is Tertullian: “It is a fundamental human right, a privilege of nature, that everyone should worship according to his own convictions.” These Christians “devised,” Ehrman writes, somewhat cheekily, “the notion of the separation of church and state.” But when Christians seized control of the empire, the separation they had long argued for vanished. The charges once lobbed against Christians — atheism, superstition — were turned against pagan people.

Ehrman is careful to note that, for the most part, there was no Christian secret police forcing pagans to convert: The empire was too large and diffusely governed to make such an effort feasible. In addition, “there was no one moment when the world stopped being pagan to become Christian.” Rather, it happened in the manner of Hemingway’s theory of bankruptcy: gradually, then suddenly. Reading about how an entire culture’s precepts and traditions can be overthrown without anyone being able to stop it may not be heartening at this particular historical moment. All the more reason to spend time in the company of such a humane, thoughtful and intelligent historian.

Tom Bissell’s most recent book is “Apostle.” His essay collection “Magic Hours” is being reissued in paperback in March.